Covalent attachment of the hydrophilic polymer poly(ethylene glycol), abbreviated PEG, is a highly advantageous method of increasing water solubility and bioavailability and extending the circulation time of many biologically active molecules, particularly hydrophobic molecules. For example, it has been shown that the water-insoluble drug, paclitaxel, when coupled to PEG, becomes water-soluble. Greenwald, et al., J. Org. Chem., 60:331-336 (1995). The total molecular weight of the polymer or polymers attached to the biologically active molecule must be sufficiently high to impart the advantageous characteristics typically associated with PEG polymer attachment, such as increased water solubility and circulating half life, while not adversely impacting the bioactivity of the parent molecule.
Proteins and other molecules often have a limited number of reactive sites available for polymer attachment. Often, the sites most suitable for modification via polymer attachment play a significant role in receptor binding, and are necessary for retention of the biological activity of the molecule. As a result, indiscriminate attachment of polymer chains to such reactive sites on a biologically active molecule often leads to a significant reduction or even total loss of biological activity of the polymer-modified molecule. To form conjugates having sufficient polymer molecular weight for imparting the desired advantages to a target molecule, prior art approaches have typically involved either (i) random attachment of numerous polymer arms to the molecule, thereby increasing the risk of a reduction or even total loss in bioactivity of the parent molecule, or (ii) attachment of one or two very long polymer chains. Unfortunately, the use of very high molecular weight linear polymer chains is problematic because of the difficulty and expense associated with their preparation, purification, and associated instability.
Branched polymers comprising a plurality of polymer arms attached to a central core and having a single reactive group for conjugation to a biologically active molecule have been described in U.S. Pat. Nos. 5,643,575 and 5,932,462. Both patents describe branched polymers formed by covalent attachment of a water-soluble polymer such as an end-capped PEG to a central core molecule bearing amino groups, such as lysine or 1,3-diamino-2-propanol. Although these branched polymers are useful for attaching a high molecular weight polymer to a molecule at a single attachment site without using an extremely long polymer chain, the methods of forming the branched PEG molecules of the prior art is difficult and requires extensive purification of the PEG polymers prior to attachment to the core molecule and also purification/removal of partially pegylated polymer intermediates.
There remains a need in the art for new branched polymer reagents that provide the benefits associated with branched polymers (i.e., high overall molecular weight in a single non-linear polymer molecule), but are easier to synthesize or provide more flexibility in their design than prior art reagents.